The present invention relates to charge pumps and, more particularly, to a charge pump feedback system for regulating the output voltage of a charge pump.
Charge pumps are devices that are capable of operating as power supplies for electronic circuitry. Charge pumps provide a controlled output voltage that is higher than the charge pump""s input voltage.
Charge pumps are often designed in stagesin order to achieve optimum efficiency of operation. Four-phased multi-stage charge pumps are generally considered to be one of the most efficient pump architectures known in the art and are, accordingly, widely utilized in the art. A co-pending patent application titled Efficient Charge Pump Apparatus and Method For Operating the Same, U.S. Ser. No. 09/827,512 filed Apr. 5, 2001, assigned to the present assignee, which is hereby incorporated in its entirety, describes several embodiments of a novel multi-staged charge pump including a 4-phase clock charge pump that is capable of providing increased voltage at its output as compared to the voltage supplied at its input. As described herein, the charge pump is designed to receive a plurality of oscillating clock signals to power and control the operation of the charge pump.
While the charge pump described in the above-referenced application achieves a certain efficiency of operation over known prior art devices, one skilled in the art will appreciate that the output of the charge pump is dependent upon many unstable variables such as VDD (the positive voltage supply), temperature, process conditions and load, and as such, requires an appropriate regulation apparatus to control and provide for a constant voltage at the output of the charge pump.
Known prior art methods of regulating the output voltage of charge pumps have varied limitations that significantly effect the overall efficiency of the charge pump""s operation.
The prior art method for providing regulation of a charge pump""s output illustrated in FIG. 1 suffers from the drawback of current inefficiency. As shown in FIG. 1, the regulated output voltage of charge pump 100 is fed into a voltage divider 102 and compared to a reference voltage (Vref) 104 at operational amplifier 106. In this prior art embodiment, the regulation of the output of charge pump 100 is achieved by shunting the output to ground 108. Thus, when charge pump 100 operates at minimum energy, e.g., low VDD, the current dissipated through ground 108 is, likewise, low. However, when charge pump 100 operates at higher energy conditions, e.g., high VDD, much more current is dissipated through ground 108, thus evidencing the inefficient operation of the regulating system of FIG. 1. Accordingly, while the system of FIG. 1 provides a regulated and constant output voltage, the charge pump may operate at a very low efficiency.
U.S. Pat. No. 5,276,646 of Kim et al., which is hereby incorporated herein by reference in its entirety, describes yet another system and method for providing a constant voltage at the output of a charge pump. The system and method taught in Kim continuously measures the output of the charge pump and enables or disables the operation of the charge pump based on the value of the measured output voltage being above or below the desired output voltage. Application of this method causes a large delay between the actual reading of the output voltage and the corrective action taken with respect to the operation of the charge pump such that a ripple effect of as high as 1 volt is introduced at the output voltage of the charge pump. Moreover, when the charge pump is turned on, large substrate noises occur.
U.S. Pat. No. 5,717,581 of Canclini, which is hereby incorporated herein by reference in its entirety, presents another method for regulating the output voltage of a charge pump whereby the output voltage is utilized to control the conductances of the charge transfer transistors along the pump which, in turn, controls the output voltage itself. This method is wasteful in terms of energy usage because, when the charge pump is under maximum energy condition, regulation of the charge pump can only be achieved by reducing the overall efficiency of the charge pump.
U.S. Pat. No. 6,107,862 of Mukainakano et al. and U.S. Pat. No. 5,559,687 of Nicollini et al., which are hereby incorporated herein by reference in their entirety, teach a system and method for regulating the output voltage of a charge pump by varying and controlling the ramp rate of the oscillating clock signals fed to the energy injection capacitors of the charge pump. Unfortunately, this method obviously limits the oscillating clock signals to low frequency operation. Moreover, varying the ramp rate of the oscillating clock signals may interfere with a 4-phase clock charge pump""s requirement for oscillating clock signals having clean, well defined, non-overlapping phases. The regulating methods and systems taught in Mukainakano and Nicollini can not effectively be utilized with 4-phase clock charge pumps (such as that described in the above-mentioned co-pending patent application) because such charge pumps require high frequency operation and non-overlapping phases.
U.S. Pat. No. 5,553,030 of Tedrow et al., which is hereby incorporated herein by reference in its entirety, describes a method for controlling the output voltage of a charge pump by varying the frequency of the oscillating clock signals that power and control the charge pump. Using this method, the output of the charge pump increases with an increase in oscillating clock signal frequency. Unfortunately, in 4-phase clock charge pumps, the output voltage of the charge pump will begin to decrease above a certain high input frequency. Thus, the use of relatively high frequencies with the system described by Tedrow will cause instability at high frequency. In order to prevent such instability, the charge pump should nominally be operated at a relatively low frequency which results in a charge pump output that is, likewise, low relative to the input. Moreover, utilization of the frequency varying system described in Tedrow requires the use of multiple oscillating clock signal circuitry to provide a signal of the particular frequency presently required at any particular node of the charge pump. This adds considerable cost, complexity and inefficiency to the design of the output voltage regulating system.
What is desired, therefore, and has heretofore been unavailable is an output voltage regulating system and method for a charge pump that avoids the inefficiencies of the above-described prior art systems while providing a stable, continuous boosted output voltage for a charge pump.
An embodiment of an improved charge pump output voltage regulating system of the present invention includes a charge pump of the type including a plurality of stages which together provide a boosted output, each stage having an energy injection capacitor charged by a clock which oscillates between a reference level (Vref) and a supply level (Vsupp), the improvement further including a feedback loop connected between the boosted output and a supply of the clock. The feedback loop dynamically regulates the supply level so that the boosted output is constant.
In a more specific embodiment of the present invention, the charge pump is part of an integrated circuit coupled to a voltage source of magnitude VDD, and the supply level (Vsupp) ranges from the reference level (Vref) to the voltage source magnitude (VDD).
In another aspect of the present invention, a charge pump having a regulated output, includes: a main pump having a driver for charging an energy injection capacitor and an output; a regulator having a first signal input connected to a constant voltage bias and a second signal input connected to the main pump output, and a signal output; and a transistor having a control terminal connected to the signal output, a voltage supply terminal connected to a voltage supply, and an output terminal connected to either a positive or a negative supply rail of the driver.
In a more specific aspect of the present invention, the transistor is an nMOS device, the control terminal is the gate terminal, the voltage supply terminal is the drain terminal, and the output terminal is the source terminal. In this configuration, the transistor is connected as a source follower.
In another aspect of the present invention, a charge pump having a regulated output, includes: a main pump having a driver for charging an energy injection capacitor and an output; a regulator having a first signal input connected to a reference and a second signal input connected to the main pump output, and a signal output; a transistor having a control terminal connected to the signal output, a voltage supply terminal connected to ground potential, and an output terminal connected to a negative supply rail of the driver.
Other objects and features of the present invention will be described hereinafter in detail by way of certain preferred embodiments with reference to the accompanying drawings.